Silver recovery, from the spent fixer solution used to develop various types of photographic films, including black and white, color and X-ray films is done for economic reasons and to prevent the discarding of hazardous waste containing silver or components of a bleach fixer solution. Current technologies for the recovery of silver from the fixer solution include metallic replacement and electrolytic plating. These have the disadvantages of high capital and maintenance costs, inefficient recovery of silver and an inability to recycle the fixer solution.
It is known how to extract silver from aqueous thiosulfate solutions using such materials as aluminum, aluminum alloys, organic phase containing a quaternary ammonium compounds and sulfide ions. In addition, publications describe the regeneration of spent photographic fixing solution using electrolysis. Some methods provide for limited fixer recycling. Representative methods in this area are as follows:
1. Morana, Simon J. "Silver Recovery from Waste Film by Burning", Precious Met. Proc. Int. Precious Met. 1981 pp. 369-377. PA0 2. Kunda, W. and Etsell, T. H. "Recovery of Silver from X-Ray Film", Precious Met, (Proc. 9th Int. Precious Met. Inst. Conf.), 1985 (Pub. 1986), 289-304. PA0 3. Kunda, W. "Processing of Photographic Spent Solution by Chemical Method", Precious Met. (Proc. 7th Int. Precious Met. Conf.) 1983, (Pub. 1984) 185-95.
Photographic processing of various types of photographic films, including black and white, color and X-ray films involves development of the image by decomposition of silver halide crystals to metallic silver and the removal of unused silver halides from the film. For example, during processing about 331/3% to 40% of the silver remains on the film and the remaining 60% to 662/3% is washed into the photographic fixer solution. Discarding of the solution is a problem in that the silver is lost and discharged as pollution into the environment. Silver is currently recovered from the fixer solution using a two stage electrolysis followed by either metallic replacement using cartridges filled with iron wires, or ion exchange to remove residual silver. Electrolysis is costly in capital outlay and in operating costs. The silver recovery is low and environmental problems remain from disposal of the effluent solution. Electrolytic plating allows for some fixer recycling, but electroplating causes a lowering of the pH of the fixer solution and consumption of both sulfite (SO.sub.3.sup.-2) and thiosulfate resulting in a decrease in the fixation rate and the amount of silver that can be stripped from the film.
Previous work has shown that compounds, such as Na.sub.2 S and H.sub.2 S, which dissociate into sulfide ions, are effective reagents for removing silver from solution in a chemical process. Problems with this process involve formation or use of poisonous hydrogen sulfide (H.sub.2 S) gas and the generation of elemental sulfur.
It is known in the prior art that the silver product can then be converted to metallic silver by heat treatment in an air atmosphere at 600.degree. C. or by dissolution in nitric acid.
A chemical process, which could selectively remove silver from spent fixer solution without destruction of the thiosulfate in solution and which would allow the fixer solution to be reused, would have the advantages of lower costs, ease of operation, and a reduced environmental hazard.